Hi i had the same problem today, after using yum-update, i was unable to login horde webmail.
The following errors occured in the messages log:
jan 14 20:01:05 server01 httpd: PHP Fatal error: Call to undefined function: get() in /home/httpd/html/horde/imp/config/prefs.php on line 40
jan 14 20:01:10 server01 httpd: PHP Fatal error: Call to undefined function: get() in /home/httpd/html/horde/imp/config/prefs.php on line 40
jan 14 20:01:18 server01 httpd: PHP Notice: Undefined index: icon in /home/httpd/html/horde/menu.php on line 37
I decided to re-run the horde3 install script and now horde works just like it used to.
"http://mirror.contribs.org/smeserver/contribs/jbennett/install_horde30.sh"
Bas